Solve for n: 3xn-9=6y

The solution set for 5v2 – 125 = 0 is

Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour from the time that she first checked her outdoor thermometer. by 6 a.m., the temperature had fallen 21 degrees. which expression can you use to find how many hours earlier she had first checked the thermometer?

With finding limits of multi variable !